Monoliths of essentially uniform density containing in situ-expandable thermoplastic particles essentially uniformly dispersed in a thermosettable or thermoplastic matrix resin system that is incompatible with the particles, are described. Composites of the monolith and core-holding structures are made by incorporating the monolith into the core-holding structure, thermally treating the matrix resin of the monolith whereby expansion of the in situ-expandable thermoplastic particles causes the thermally treated monoliths to faultlessly interface with the wall(s) of the core-holding structures. The invention relates to core supported composite structures, fasteners, adhesives, paneling, insulation, and other structures that employ the invention as a component.
Sandwich construction forms a major part of aerospace manufacturing; it is employed to some degree in almost every type of flight vehicle. Lightweight structural panels and panel systems for a wide variety of applications are industrially offered. They utilizing various laminating techniques to adhere thin, stiff xe2x80x9cskinsxe2x80x9d, such as aluminum, steel, hardboard and fiberglass onto lightweight core materials, like honeycombs, urethane foams and balsa. The resulting panel is lightweight and strong.
Honeycomb sandwiches, i.e., honeycombs, are preferred structures in the fabrication of lightweight structures typically used in the aerospace and other commercial markets. The core material is usually xe2x80x9csandwichedxe2x80x9d between skins of aluminum or other high strength composite material. A bonding adhesive is used to attach the xe2x80x9cskinxe2x80x9d material to the honeycomb core. The resultant honeycomb panel offers one of the highest strength to weight constructions available. For instance, the floor panels of most airliners use the lightweight/high strength construction of honeycomb. Aircraft engine nacelles, flaps, overhead bins and galleys all are constructed from honeycomb core.
A honeycomb sandwich, or simply xe2x80x9choneycomb,xe2x80x9d may be called a multicellular structure, and it may be made of paper, plastic, fabric or metal, and other materials. The core of the sandwich is the honeycomb, a structure composed of row upon row of framed cells, or holes or wells, resembling the honey-storage facility of a beehive and characterized by a hexagonal or rectangular shape. To each side of the core are bonded extremely thin sheets of metal, creating the sandwich, which is far lighter yet has greater resistance to bending than a comparable thickness of metal plate. Aluminum is the most extensively used metal, in both the core and the facing sheets, but the technique is applicable to a large number of metallic and nonmetallic materials.
For example, M. C. Gill Corporation offers the following honeycomb products to Boeing""s specifications:
To illustrate some of the physical characteristics of commercial honeycombs, the following is taken from Plascore Inc. (615 North Fairview Ave. Zeeland, Mich. 49464) PAMG-XR1 5052 Aluminum Honeycomb literature:
Other Plascore honeycomb products include:
PAMG-XR1 5056 aluminum honeycomb is a lightweight core material which offers superior strength and corrosion resistance over PAMG-XR1 5052 and PCGA Commercial grade aluminum core. PAMG-XR1 5056 core is made from 5056 aluminum alloy foil and meets all the requirements of MIL-C-7438.
PCGA-XR1 honeycomb is a lightweight core material offering excellent strength and corrosion resistance for industrial applications at low cost. PCGA-XR1 core is made from 3003 aluminum alloy foil.
PN Aramid Honeycomb is a lightweight, high strength, non-metallic honeycomb manufactured with aramid fiber paper (DUPONT Nomex((trademark)) or equivalent). The aramid paper is treated with a heat resistant phenolic resin. This core material exhibits excellent resiliency, small cell size, low density and outstanding flame properties.
Plascore polycarbonate honeycomb core exhibits a unique cell structure: The core has 3 orientations vs. the 2 orientations common with other cores, making its properties more uniform. Each cell has a tubular form and inherently stable.
Plascore polypropylene honeycomb core exhibits a unique cell structure: The core has 3 orientations vs. the 2 orientations common with other cores, making its properties more uniform. Each cell has a tubular form and inherently stable.
Plascore polypropylene honeycomb is supplied with or without a non-woven polyester veil for better bonding. It is also supplied with or without a film barrier under the polyester veil to limit the amount of resin consumption.
Euro-Composites(copyright) sell honeycombs with cell sizes ranging from 3.2 to 19.2 mm and a density of between 24 and 200 kg/m3, in hexagonal and rectangular-celled cores.
Aerospace manufacturers started the use of honeycomb products in airplanes and spacecraft because, pound-for-pound, it""s the strongest, most rigid product known. Some of its special uses were the heat shield on John Glenn""s space capsule, interior structures for America""s first Skylab, and shock-resisting hulls of hydroplanes.
There are many ways to fasten one structure to another. Fasteners are objects that attach one item to another, and create methods by which those attachments are effected. In industrial applications, fastening may be accomplished through needle stitching, anchoring, connecting, locking, welding, riveting, nailing, screwing, adhesive bonding, chemical reaction bonding, magnetic bonding, and the like.
The concept of fastening started with the earliest concepts of interweaving of dissimilar materials, clamps, nailing, screwing and the like. Eventually, man learned to bond with metals, and this led to riveting, bolting and welding. Each of these techniques led to advances in the art of fastening.
An adhesive is a substance used to bond two or more surfaces together. Most adhesives have the advantage of forming a bond by filling in the minute pits and fissures normally present even in very smooth surfaces. Adhesive bonds are economical, distribute the stress at the bonding point, resist moisture and corrosion, and eliminate the need for rivets and bolts. The effectiveness of an adhesive depends on several factors, including resistance to slippage and shrinkage, malleability, cohesive strength, and surface tension, which determines how far the adhesive penetrates the tiny depressions in the bonding surfaces. Adhesives vary with the purpose for which they are intended. Such purposes now include the increasing use of adhesives in aerospace applications. Synthetic adhesives used both alone or as modifiers of natural adhesives, perform better and have a greater range of application than the natural products. Most of them form polymers, huge molecules incorporating large numbers of simple molecules to form strong chains and nets that link surfaces in a firm bond. Thermosetting adhesives, which are transformed into tough, heat-resistant solids by the addition of a catalyst or the application of heat, are used in such structural functions as bonding metallic parts of aircraft and space vehicles. Thermoplastic resins, which can be softened by heating, are used for bonding wood, glass, rubber, metal, and paper products. Elastomeric adhesives, such as synthetic or natural rubber cements, are used for bonding flexible materials to rigid materials.
Many aerospace structures are adhesively bonded through the use of thin adhesive films, typically made from a filled thermosetting resin such as an epoxy resin. These films are easier to apply and cleaner to use, and therefore find wide acceptance in applications where neat utilization of the adhesive is a plus factor.
SynCore(copyright) sold by The Dexter Corporation, Aerospace Materials Division, 2850 Willow Pass Road, Bay Point, Calif. 94565, is a syntactic foam film that takes the place of more expensive prepreg plies in stiffening critical structures. This isotropic foam is a composite material containing preformed microballoons in a thermosetting matrix resin. A wide variety of preformed microballoons and matrices can be combined to make SynCore(copyright) materials. Glass is the most common microballoon material of construction, but quartz, phenolic, carbon, thermoplastic and metal-coated preformed microballoons have been used. Epoxies curing at 350xc2x0 F. (177xc2x0 C.) and 250xc2x0 F. (121xc2x0 C.) are the most common thermosetting matrix resins, but matrices of bismaleimide (BMI), phenolic, polyester, PMR-15 polyimide and acetylene or acrylic or vinyl-terminated resins have been used to produce SynCore(copyright) syntactic foams. As a result of the variety of materials that successfully make SynCore(copyright), they are tailorable to a variety of applications. There is a version of SynCore(copyright) available that will co-cure with all known available heat-cured composite-laminating resins. SynCore(copyright) allows sandwich core concepts to be used in a thinner dimension than previously possible. The thickness limit on honeycomb cores is approximately 0.125, inch. SynCore(copyright) is available in 0.007 to 0.125. inch (0.18 mm to 3.2 mm) thickness but can be made in thinner or thicker sheet forms. Other core materials such as wood and sheet foam can be made thin, but are not drapable and generally require an expensive/heavy adhesive film to bond to the partner composite components. In addition, SynCore(copyright) possesses excellent uniformity in thickness which provides the ability to assure quality for the composite in which it is used as a component. SynCore(copyright) is typically used to replace prepreg plies where the intent is to increase stiffness by increasing thickness.
Designing with SynCore(copyright) is straightforward because all of the analysis methods that apply to other core materials such as honeycomb apply to it. Flexural stiffness of flat plates and beams increases as a cubic function of thickness allowing a lighter, stiffer lamination than could be made from prepreg plies alone. Since SynCore(copyright), on a per volume basis, typically costs less than half of a comparable carbon prepreg, it also leads to a lower cost lamination. This is illustrated by the following:
1. Adding one ply of 0.020 inch SynCore(copyright) and eliminating one ply of prepreg does not change the weight or cost significantly, but nearly doubles the flexural rigidity.
2. Adding one ply of 0.020 inch SynCore(copyright) and eliminating three plies of prepreg sharply decreases the cost and weight with a small decrease in rigidity.
3. Adding one ply of 0.040 inch SynCore(copyright) and eliminating three plies of prepreg provides lower weight, cost and sharply increases rigidity.
4. The introduction of unidirectional tape allows a further increase in performance at lower cost and weight at nearly the same thickness.
5. A hybrid tape/fabric/SynCore(copyright) construction gives a very attractive set of weight and cost savings coupled with a 3.4 times increase in flexural rigidity.
SynCore(copyright) has been recommended for thin composite structures in any application where flexural stiffness, buckling, or minimum gauge construction is used. It has been shown to save weight and material cost in carbon fiber composites. It has been offered to save weight at approximately the same cost in the case of glass fiber composites. Illustrative applications are covered in U.S. Pat. No. 4,861,649, patented Aug. 28, 1989, U.S. Pat. No. 4,968,545, patented Nov. 6, 1990, and U.S. Pat. No. 4,994,316, patented Feb. 19, 1991.
The manufacturing methods for employing SynCore(copyright) are very similar to those used for prepregs. Because it is not cured, it is tacky and very drapable when warmed to room temperature and is easier to lay-up than a comparable prepreg ply. It can be supplied in supported forms with a lightweight scrim to prevent handling damage when it is frozen. It requires cold storage like prepregs, usually 0xc2x0 F. (xe2x88x9217.7xc2x0 C.) or below. The various SynCore(copyright) materials typically have a room temperature out-time that is much longer than their companion prepregs. SynCore(copyright) is less sensitive to cure cycle variations than prepreg making the controlling factor the composite cure cycle selection. It will cure void free under full vacuum or low (e.g. about 10 p.s.i.) autoclave pressure. It has been cured at up to about 150 p.s.i. without exhibiting balloon crushing.
In a typical application, a sandwich of SynCore(copyright) and prepreg, such as a thicker layer of SynCore(copyright) between two thinner layers of prepreg, are held together under heat and pressure to cure the structure into a strong panel. Typical sandwich constructions of this nature are shown in U.S. Pat. Nos. 4,013,810, 4,433,068 and 3,996,654. Such composite structures typically are produced in flat sheets and in separable molds to obtain various desired shapes.
Though SynCore(copyright) will cure void free under significantly reduced pressure or when put under pressure, it would be desirable to avoid those costly conditions to achieve void reduction. It would be desirable to have a material that has the properties of SynCore(copyright) but achieves void free construction without costly full vacuum operations or low autoclave pressure systems. These methods are typically batch type operations that materially add to the cost of making the composite.
There are certain applications in which it is desirable to have the properties of a uniform thin drapable syntactic foam film in processing the formation of a laminated composite, yet have the capacity to autogenously expand so as to fill any void space existing in the composite""s structure so as to minimize the effects of macro and micro void defects at interlaminate interfaces.
These interlaminar interfacial micro or macro void spaces are magnified by the irregularity of the reinforcing layer of the composite structure. For example, if the composite is of a layer of prepreg-derived carbon fiber reinforced thermosetting resin material, bonded to a syntactic foam, such as a SynCore(copyright) thin uniform film, the layer containing the prepreg-derived material will have an irregularly shaped surface and the SynCore(copyright) layer will have a relatively smooth uniform surface. Though the SynCore(copyright) is tacky and drapable, it is incapable of filling in all of the irregularities of the prepreg-derived layer. Application of a full vacuum or the use of a low-pressure autoclave can be used to significantly reduce the void space, but complete avoidance of micro voids is not readily achievable. Also, conforming SynCore(copyright) to the irregular surface causes transfer of the irregularity to the opposite surface of the SynCore(copyright) film. Such surface irregularity transfer may be avoided by sandwiching the SynCore(copyright) film using heat and pressure; such repositions the film""s matrix resin and the microspheres so that the film within the sandwiched structure loses its original uniformity.
It would be desirable to be able to adequately bond a syntactic foam thin film, to an irregular surface1 and fill the defects in the surface without transferring the shape of the defects to the unbonded side of the film. It would also be desirable to be, able to adequately bond a syntactic foam thin film to a surface and, without the use of vacuum or low-pressure autoclaves, fill the micro voids with the syntactic foam without repositioning the film""s matrix resin and microspheres. Such advantages are achieved by the use of SynSpand(copyright), described below.
1 Such a surface is one that may contain undulations, cracks, large pores, warpage, and the like defects. 
There is a body of technology directed to fabricating expandable thermoplastic resinous, material. For example, U.S. Pat. No. 2,958,905, patented Nov. 8, 1960, is directed to a method of making foam structures from particulate expandable granular thermoplastic resinous material containing in the particles a blowing agent for further expansion of the particles. A considerable number of thermoplastic resins are described as suitable for this purpose. The blowing agents are the conventional ones recommended for that application. The expandable granular thermoplastic resinous material may be admixed with a thermosetting resin to generate on curing the exotherm needed to expand the expandable granular thermoplastic resinous material. The resulting mass can be poured into a mold to make a number of products. The patentees indicates that the expandable granular thermoplastic resinous material can be formed in the presence, of non-expandable filler materials such as staple fibers from a variety of sources, and the mixture fed to a mold for forming an expanded product. The resulting foamed product may be designed to adhesively bond to a fabric layer for reinforcement of the foamed product. The density of the foamed product can be controlled by the amount of the expandable material fed to the mold. According to the patentees, starting at column 12, lines 5 et seq., molded products are formed by charging the mold xe2x80x9cwith the expandable material in any desired manner including manual filling or pneumatic conveyance thereof.xe2x80x9d According to the description at column 12 relating to FIGS. 3 and 4 (see column 12, lines 16-32):
. . . xe2x80x9ca considerable occurrence of void and hollow spaces occurs between the charged expandable beads 21 in the mass to be fabricated, each of which (in the case of pre-expanded material) is a foam structure containing a plurality of internal cells or open spaces. When the liquid exothermus [sic] substance is added between such interparticle voids, the heat from its spontaneous self reaction causes the beads to expand whereby, as illustrated in FIG. 4, the expanded and fabricated particles 22 force out a substantial portion (and frequently most) of the exothermus [sic] substance excepting for a minor quantity of reacted material 23 which remains, frequently as an interlaced and interlinking network between the expanded particles to assist in holding the expanded, cellular foam particles together.xe2x80x9d (Emphasis added)
U.S. Pat. No. 2,959,508, patented Nov. 8, 1960, describes another variation of using expandable thermoplastic particles. In this patent, the unexpanded particles and the exothermus substance, such as an epoxy resin, are first mixed and then poured into the mold to form a composite foam of the two when the exothermus substance heats up the mixture and causes the blowing agent to volatilize.
Thermosetting resins have had blowing agents incorporated in them (see U.S. Pat. No. 3,322,700, patented May 30, 1967) to form expanded molded products and recently, such types of resin systems have included preformed microspheres in the formation of partial syntactic foam films. These expanded thermosets comprise a more open cellular structure unlike that of syntactic foams, and the inclusion of preformed microspheres does not alter that condition.
There are commercial molding processes that utilize tacky sheets of thermosetting resins and reinforcing material. One such process involves the compression molding of sheet molding compounds (xe2x80x9cSMCxe2x80x9d). In that process, a thermosetting polyester resin filled with staple glass fiber and low profile thermoplastics, are sheeted out and thickened into a pourable paste retained between release surfaces such as polyethylene film. Chunks of the thickened paste are casually deposited around the surface of the mold by hand, and on closing the mold with heating, the paste is liquefied and it, and its fiber loading, are redistributed around the mold to fill it up and form the desired molded article. In other word, the chunks of sheets of SMC represent a convenient way in which to add a liquefiable moldable material to the mold. This process is presently commercially practiced in a number of industries. Advantages of the process are the convenience of storing moldable mixture and the ease of loading a mold with the molding composition.
An advantage of SynCore(copyright) for many applications resides in its uniformity of distribution of the microsphere throughout the matrix resin. Such microspheres remain essentially intact throughout the cure cycle. As a result, it is not possible to have the microspheres concentrate at one or more surfaces, or one or more other locations in the final composite. It would be desirable to have a drapable thin film, having the handling qualities of SynCore(copyright), but which would allow the production of a syntactic foam having a controllable density gradient that accommodates specific end use applications.
There are a number of applications in which thin film syntactic foam could serve as a seal to preclude the passage of gases and liquids. In some applications, the seal could be subjected to abrasion forces. It would be desirable to have a thin film syntactic foam that can be applied in a manner that allows it to be a sealant to gas or liquid flow in a confined space and be able to withstand abrasive forces. Such advantages are derived from the use of SynSpand(copyright), described below.
U.S. Pat. Nos. 5,234,757; 5,397,611; 5,540,963; and 5,783,272 describe a thin film technology that can be used in forming syntactic foam. The thin film is commercially sold as SynSpand(copyright) by the Dexter Corporation, Dexter Adhesive and Coating Systems, Bay Point, Calif. 94565. It combines expansion with syntactic technology to produce an efficient core filler and reinforcement material. It comprises a thin, tacky film of incompatible in situ-expandable thermoplastic particles in a thermosettable matrix resin that contains an essentially uniform density and thickness across the breadth of the film. In its optimum form it provides closed cell expansion, see U.S. Pat. No. 5,783,272. The thin unexpanded films can be laid up in a honeycomb core and each film that is laid up is homogeneously expanded therein to reinforce and stiffen the honeycomb structure at that portion of the structure, without unduly adding weight to the structure. It is sold in 1 foot by 2-feet sheets and rolls stock (18 inches by 25lineal feet) in thicknesses of 50 and 100 mils. SynSpand(copyright) may contain preformed microspheres as well, such as those that are used in making SynCore(copyright).
SynSpand(copyright) is an excellent product and has gained wide commercial acceptance. However, as the utilizations of such forms of syntactic foams are expanded, there are uses where the thin film qualities of SynSpand(copyright) do not conveniently meet the needs of the user. For example, in honeycomb applications, a sheet or sheets of SynSpand(copyright) are applied over a plurality of honeycomb open spaces in the core, and with heat and pressure, generally under vacuum, but preferably by use of positive pressure from a heated platen, the SynSpand(copyright) is forced into open spaces to which it is contiguous. This requires the SynSpand(copyright) to flow into the honeycomb structure and assume some dimensional structural similarity to that of the open space of the honeycomb into which it flows. This technique insures that the SynSpand(copyright) is in contact with the honeycomb surface over which it is originally laid, either the top or bottom surface of the honeycomb structure, and the walls of the open cells of the honeycomb structure. It is inevitable that some amount of the SynSpand(copyright) film is left behind on that surface. Further heating cause the matrix resin to flow and sag into the core of the honeycomb structure, and the in situ-expandable particles to expand, preferably before gelation of the matrix resin, primarily in the xe2x80x9czxe2x80x9d direction (see the discussion in the bridging paragraph at columns 9 and 10 of U.S. Pat. No. 5,397,611), and allows for curing the expanded SynSpand(copyright) in the honeycomb. This serves to reinforce and stiffen the honeycomb structure without unduly adding weight to the structure. However, the small amounts of SynSpan(copyright) left on the top or bottom surface of the honeycomb are going to be expanded as well. In addition, sagging of the resin causes it to contact the walls of the honeycomb core before expansion, thus expansion not only is in the xe2x80x9czxe2x80x9d direction, but also starts from the wall to fill the center of the core; which is opposite to the type of expansion that one would desire. The small amount of SynSpand(copyright) left on the top or bottom surface can be ameliorated to some degree by applying an adhesive film over that surface into which the residual SynSpand(copyright) can be solubilized.
A deficiency in thin film SynSpand(copyright) technology resides in the fact that the top of the film is the primary exposed surface area for expansion. Because the edges of the film possess relatively little of the films surface area, and the molding method of choice forces the edges into contact with the walls of the open cells of the honeycomb structure followed by sagging, expansion must of necessity occur essentially from and in the xe2x80x9czxe2x80x9d direction and also from the core walls. This concentration of expansion to essentially a single surface fails to optimize the strength/density/rigidity relationship in such honeycomb structures and does not readily allow reinforcement within the open cells of the honeycomb structure by continuous filament fibers aligned normal to the surface of the honeycomb structure. Under the sagging scenario, expansion in the xe2x80x9czxe2x80x9d direction causes the resin to intersect at some part of the open space being filled. Unless the resin contact is seamless by virtue of molten mixing of the different resin film, there is an opportunity for the resin to form an interface within the open space that possesses a lower structural integrity than the remainder of the expanded polymer structure. In addition, thin film SynSpand(copyright) technology does not readily allow structural combinations with continuous filament structures in a way that maximizes rigidity and strength to syntactic foam structures incorporated into a traditional honeycomb structure.
This invention relates to solid adhesive structures, core fillers and reinforcement materials, and fasteners that rely on syntactic foam technology and are suitable for use in many different industries, such as aerospace, automotive, building construction, and the like. More specifically, this invention relates to reinforcement or stiffening of a normally open-cellular structural material (such as any honeycomb structures (such as those described above), any tubular structures, any channel structures, and the like). The invention is an expandable sag-resistant nucleus-forming forming monolithic composite that can be located within a hollow interior portion of the aforementioned structural material. This invention involves forms of a pre-shaped expandable sag-resistant nucleus-forming monolithic composite that can be used more conveniently and function more effectively in reinforcing and stiffening a normally open-cellular structural material. The invention relates to structural material containing at least one of the expandable sag-resistant nucleus-forming monolithic composites located within a hollow interior portion of the structural material. The invention relates as well to processing for making and using the pre-shaped expandable sag-resistant nucleus-forming monolithic composite of the invention.
The pre-shaped expandable sag-resistant nucleus-forming monolithic composite comprises incompatible in situ isotropically expandable thermoplastic particles containing expansion agent therein, essentially uniformly distributed in a thermosettable or thermoplastic matrix resin with which they are incompatible when the thermoplastic particles are in the thermo-expandable state. Where the monolithic composite contains a thermosettable matrix resin, the resin is not fully cured or at such a state of cure that it inhibits the desired degree of expansion of the composite. Preferably, the resin is devoid of a degree of crosslinking that exhibits viscosity increase in the composite. The composite contains an essentially uniform density and thickness across its breadth and it possesses an external shape and size that is dimensionally similar to a hollow interior component of the structural material.
xe2x80x9cPre-shapedxe2x80x9d means that the monolithic composite does not undergo a shaping process when introduced into the hollow interior component of the structural material. It is already shaped for introduction into the hollow interior component. The term xe2x80x9cdimensionally similarxe2x80x9d means that when the nucleus-forming composite is placed in the hollow interior component of the structural material and is heated sufficiently to cause isotropic expansion of the incompatible in situ-expandable thermoplastic particles, the nucleus-forming composite uniformly expands to cause the formation of a syntactic foam, especially a closed cell syntactic foam, that has an essentially faultless interface with the wall(s) of the hollow interior component and possesses an essentially uniform density throughout. As used herein, the term xe2x80x9cmonolithicxe2x80x9d means something that is essentially uniform and substantial or inflexible in quality or character. The term xe2x80x9cthermo-expandable statexe2x80x9d means the in situ-expandable thermoplastic particles heated to a condition where the expansion agent in the particles are expanding; this means that the thermoplastic resin in the particles are sufficiently softened that the particles expand in size. The term xe2x80x9csag-resistantxe2x80x9d means to resist sinking or bending, by or as if by weight or pressure when the composite is placed in the hollow interior component of the structural material, sufficiently to avoid snagging of the composite on the hollow interior. The term xe2x80x9cfaultlessxe2x80x9d means that the flaws inherently present in the wall are filled and sealed by the expanded composite. The term xe2x80x9cessentially uniformxe2x80x9d means that the product has a prescribed uniformity according to a predetermined standard.
This invention relates to a sag-resistant nucleus-forming monolithic composite of incompatible in situ-expandable thermoplastic particles that form closed-microcells upon isotropic expansion, essentially uniformly distributed in a thermosettable or thermoplastic matrix resin. The monolithic composite contains an essentially uniform density and thickness across its breadth. Also the monolithic composite possesses an external shape and size that is dimensionally similar to a hollow interior component of a structural material that is about 1.01 to about 4 times greater in volume than the volume of monolithic composite. In other words, the sag-resistant nucleus-forming monolithic composite of the invention has a shape and has dimensions that, in a preferred embodiment, essentially correlates the shape and is at least close to proportional to the dimensions of the hollow interior component of a structural material into which it can be readily inserted and isotropically expanded to form a syntactic foam with closed microcells. The conformance of the shape and dimension of the sag-resistant nucleus-forming monolithic composite of the invention to the hollow interior component of a structural material is significant to achieve the advantages of the preferred invention. The proportionality of the dimension of the sag-resistant nucleus-forming monolithic composite to the hollow interior component is such that the composite isotropically expands about 1.01 to about 4 times in volume to fully fill the previously hollow interior component. In the preferred embodiment, the end surface of the monolithic composite is appropriately shaped such that it can stand within the hollow interior component without touching any of the sidewalls of the hollow interior component.
Sag-resistance is an important feature of the sag-resistant nucleus-forming monolithic composite. Sag-resistance must exist at the time the monolithic composite is inserted in the hollow interior component of the structural material. Sag-resistance is commensurate with the dimension similarity of the monolithic composite to that of the hollow interior component. The closer the dimension similarity, the greater must be the sag-resistance. In other words, the monolithic composite must be capable of insertion into the hollow interior without snagging of the composite on any surface of the hollow interior during insertion. Preferably, the monolithic composite should have enough sag resistance that the composite does not sag once it is placed in the hollow interior component of the structural material.
The term xe2x80x9cisotropic expansionxe2x80x9d as use herein means the expansion along all axes or directions up until a surface portion(s) of the monolithic composite during expansion makes contact with and forms an essentially faultless interface with the wall(s) of the hollow interior component. For example, the hollow interior component may have an irregular surface such that a portion of the monolithic composite""s outer surface may be dimensionally closer to a juxtaposed portion of the irregular surface. During expansion such closer outer surface may make faultless interface with the closer wall portions before other portions of the outer surfaces make faultless interface with other juxtaposed wall portions of the hollow interior component. In many instances, the monolithic composite may be placed in contact with a segment of the wall surface of the hollow interior component before expansion is initiated. In those instances, xe2x80x9cisotropic expansionxe2x80x9d continues but the axis or directions may be reoriented such that multiple isotropic expansions occur within the expanding monolithic composite. The overall effect of such expansion is that the expanded monolithic composite possesses an essentially uniform density throughout.
The uniform density of the syntactic foam of the invention derived from isotropic expansion of the monolithic composite means that the density in any direction is essentially uniform, that is, the density does not differ by more than about xc2x15 percent (%) in weight in any direction. However, density differences can be intentionally built into the resulting syntactic foam. For example, one may include reinforcing fibers or fillers that are located at preferred segments of the resulting syntactic foam so that the resulting syntactic foam composite has increased reinforcement at those segments.
An important alternative embodiment, of the invention is a fiber reinforced sag-resistant nucleus-forming monolithic composite of incompatible in situ-expandable thermoplastic particles in a thermosettable or thermoplastic matrix resin that contains an essentially uniform density and thickness across the breadth of the composite and which possesses an external shape and size that is dimensionally similar to a hollow interior component of a structural material. In this embodiment, the fiber may be present as continuous filament, continuous tow, chopped or staple fibers, spunbonded fibers, fibrous mat or fibrous webs, or any combination of them. The fibers may be aligned essentially with the aspect orientation of the monolithic composite, assuming that the monolithic composite possesses an aspect orientation. In many instances, the fibers may be aligned in a single direction or more than one direction. For example, a filamentary tow may be used that has a twist, thus imposing an alignment in several directions all within the same fibrous component of the composite. The fiber reinforcement may be of organic or inorganic fibers. In the preferred invention, at least a portion of the fiber reinforcement is, made of a fiber that conducts heat better than the thermosettable matrix resin of the composite. Most preferably, the fiber reinforcement is made of a metal or carbon-based material, such as steel, aluminum, graphite, non-graphitic carbon (including pitch based fibers), and can include high performance fibers such fibers from polyarylamides and polyarylimides, polyaromatic ether ketones such as PEEK, PEK and/or PEKE, and the like. In a preferred embodiment, the fiber is a filamentary tow that is align centrally of the interior of the monolithic composite and extends from the bottom to the top of the monolithic composite.
The monolithic composite may also contain reinforcing fillers that are materials, which enhance the toughness or tensile strength of the resulting cured, expanded product. The fillers may be of any of the known types that are used in thermosetting resins for the enhancement of toughness and/or tensile strength in cured thermosetting resins. Fillers may be included solely or complimentarily for the purpose of altering the viscosity of the monolithic composite.
The invention relates to molding structures which function as solid adhesive structures, core fillers and reinforcement materials, and/or fasteners that rely on syntactic technology and are suitable for use in many different industries, such as aerospace, automobiles, building construction, and the like. A preferred application of the sag-resistant nucleus-forming monolithic composite of incompatible in situ-expandable thermoplastic particles in a thermosettable or thermoplastic matrix resin that contains an essentially uniform density and thickness across the breadth of the composite and which possesses an external shape and size that is dimensionally similar to a hollow interior component of a structural material, is to formulate the matrix resin to be an adhesive such that it adhesively bonds to the wall(s) of the hollow interior component and forms an essentially faultless adhesive interface with the wall(s) of the hollow interior component. Because of the shape similarity of the sag-resistant nucleus-forming monolithic composite of incompatible in situ-expandable thermoplastic particles in a thermosettable or thermoplastic matrix resin of the invention, it is possible to produce solid adhesive structures, core fillers and reinforcement materials, and/or fasteners that are relatively easier to employ and which provide surprisingly better functionality. For example, it is described in U.S. Pat. Nos. 5,234,757; 5,397,611; 5,540,963; and 5,783,212 that the syntactic foam films that are now commercially sold as SynSpand(copyright), may be multi-plied into larger structures that can fill up, e.g., an open space (or hollow core or open cell) of a honeycomb aerospace structure. However, because of the sagging characteristics of the SynSpand(copyright), it is not possible to readily form a moldable structure that conforms to the dimensions of the open space or core that does not snag when introduced to the open space or hollow core of the honeycomb structure. Secondly, multi-plied structures are incapable of having variability in the reinforcement built into the plied structure without making different thin layers of SynSpand(copyright) with different reinforcement properties and plying such layers. Moreover, each of the layers must be laid up prior to inserting the multi-plied structure into the hollow core. That is a time consuming activity and the resulting plied structure must be caused to lose its pliability in order to avoid snagging on insertion into the hollow interior. In addition, SynSpand(copyright) layers that are extruded into a honeycomb take extensive time from lay-up of the layers, to the extrusion and curing of the SynSpand(copyright) within the honeycomb. As pointed out above, SynSpand(copyright) when shaped within a open cell of a honeycomb, will effect most of its expansion in the xe2x80x9czxe2x80x9d direction because the SynSpand(copyright) is drawn into the open cell and is seized by the walls of the open cell. Expansions in the xe2x80x9cxxe2x80x9d and xe2x80x9cyxe2x80x9d directions are inhibit by the walls.
This invention effects reinforcement and/or stiffening of a structural member that contains a hollow interior portion. This is accomplished by forming a sag-resistant nucleus-forming monolithic composite of incompatible in situ-expandable thermoplastic particles in a thermosettable or thermoplastic matrix resin, that contains an essentially uniform density and thickness across the breadth of the composite and which possesses an external shape and size that is dimensionally similar to a hollow interior component of the structural member, for example, a honeycomb structure. This allows the monolithic composite to be inserted into the hollow interior portion of the structural member, such as a hollow cell of a honeycomb structure, without filling up the hollow interior portion. If the hollow interior portion contains more than one opening for inserting the composite, then that opening is closed off such as to allow the monolithic composite to reside wholly in the hollow interior portion. Then the hollow interior portion is heated sufficiently to cause the matrix resin of the monolithic composite to flow and the in situ-expandable thermoplastic particles to simultaneously isotropically expand whereby the composite uniformly expands in essentially all directions (i.e., the xe2x80x9cxxe2x80x9d, xe2x80x9cyxe2x80x9d and xe2x80x9czxe2x80x9d directions) or essentially in the xe2x80x9cxxe2x80x9d and xe2x80x9cyxe2x80x9d directions, to cause the formation of a syntactic foam, especially a closed cell syntactic foam, that has an essentially uniform microcellular structure and a more faultless interface with the wall(s) of the hollow interior portion of the structural member, e.g., the honeycomb structure. In other words, it is the viscosity reduction of the matrix resin that allows the expansion agent in the in situ-expandable thermoplastic particles to force expansion of the thermoplastic resin in which it is embedded. With the matrix resin phase changes, there is a commensurate expansion of the expansion agent in the expandable particles, to where expansion is finally constrained by the matrix resin reaching a state of gelation that puts a constraint on expansion agent expansion. When the matrix resin is a thermosetting resin, it is allowed to cure to form a degree of stiffness that is sought for the syntactic foam. In the case where the matrix resin is a thermoplastic resin, expansion is constrained by the pressure of the thermoplastic resin and the collision of the expanding particles. An adhesive layer can be placed over the opening to the hollow interior portion during such expansion whereby the expanded monolithic syntactic foam composite material adhesively and integrally bonds to the adhesive layer. If desired, a metal or plastic layer can be placed over the opening to the hollow interior portion during such expansion and the expanded monolithic syntactic foam composite material adhesively and integrally bonds to the metal or plastic layer. If desired, a sealing plate that does not bond to the expanded monolithic syntactic foam composite material can be used instead. It can be removed after expansion is complete.
Another embodiment of this invention is a sag-resistant nucleus-forming monolithic composite of incompatible in situ-expandable thermoplastic particles in a thermosettable or thermoplastic matrix resin that is filament fiber reinforced and can be adhesively anchored in a hollow interior of structural member. In particular, this embodiment contemplates creating at least one hollow interior that extends through a plurality of layers of a composite and fills such hollow interior with the fiber reinforced sag-resistant nucleus-forming monolithic composite of incompatible in situ-expandable thermoplastic particles in a thermosettable or thermoplastic matrix resin, of the invention, and through thermal expansion of the monolithic composite, forming adhesive anchoring in each layer of the composite through which the hollow interior extends, thereby reinforcing and fastening: the composite into a reinforced structure. This embodiment of the invention is especially applicable to use in reinforcing honeycomb structures by including in the monolith composite the filament fiber component such that it is aligned from surface to surface of the honeycomb structure, and through adhesive layers provided onto such surfaces prior to the curing and expansion of the monolithic composite, the filament fibers become bonded to the adhesive layers, thereby greatly reinforcing the strength of the honeycomb and any film that is bonded to the adhesive layers on the side thereof oriented away from the honeycomb structure. Sandwich structures having great strength and utility can be made comprising outer layer (such as films of metal, plastic, paper, fiber reinforced layers, and the like, followed by an uncured adhesive layer, the honeycomb filled (in whole or part) with the uncured and unexpanded monolithic composites of the invention, another adhesive layer, another similarly filled honeycomb layer, an additional adhesive layer, and lastly, another outer layer. This sandwich structure can be subjected to heat and pressure so as to facilitate the isotropic expansion and curing of the monolithic composites and the curing of the adhesive layers. This will produce a multifaceted composite having great utility in a variety of area, such as in aerospace, automotive structures, building structures and the like.
A further embodiment of the invention relates to advancing the reinforcing and fastening of a composite as set forth in the preceding paragraph. In this further embodiment, the sag-resistant nucleus-forming monolithic composite of incompatible in situ-expandable thermoplastic particles in a thermosettable or thermoplastic matrix resin that is introduced into the common hollow interior of the composite to be joined, has at least one end portion that extends outside of the composites external surface, and during heat expansion of the monolithic composite, the end portion is allowed to expand greater than the confinement space limiting expansion that occurs within the hollow interior. That end portion can be pressed during the curing process to force the resin and reinforcement fibers thereat to expand beyond the size of the diameter of the opening of the hollow interior at the composite""s external surface. By applying a closed pressure on the end portion, it is flattened to some degree to form an expanded syntactic foam fiber-reinforced fastener head terminating the syntactic foam derived by heat expansion of the sag-resistant nucleus-forming monolithic composite of incompatible in situ-expandable thermoplastic particles in a thermosettable or thermoplastic matrix resin.
Fastening pressures can be increased in fiber-reinforced syntactic foam formed by the heat expansion and curing of the sag-resistant nucleus-forming monolithic composite of incompatible in situ-expandable thermoplastic particles in a thermosettable matrix resin by utilizing a matrix resin formulation that shrinks during cure, thereby causing the fiber reinforcement to become more taut within the syntactic foam.
The advantage of such a fastener structure is the fact that only one end of the hollow interior need extend through the surface of the composites outer surface. Also, anchoring of the syntactic foam of the invention into the hollow interior can be enhanced by varying the diameter of the hollow interior such that the syntactic foam is allowed to be more expanded in portions of the hollow interior, especially at a distance from the open end of the hollow interior exiting the outer surface of the composite being fastener-reinforced.
In a further embodiment of the invention, it is desired that during the process of converting the sag-resistant nucleus-forming monolithic composite of incompatible in situ-expandable thermoplastic particles in a thermosettable or thermoplastic matrix resin into a syntactic foam that the temperature applied be sufficiently high enough to cause expansion of the thermoplastic particles prior to the gelation of the resin matrix. This allows the in situ expandable particles to expand in a matrix that normally before gelation will have lost some viscosity thus favoring the expansion process, and which upon gelation will impede the, expansion of the in situ expandable particles by the increase in viscosity.
This invention relates to a process for reinforcing and/or stiffening a honeycomb structure which comprises inserting into a select hollow interior portion (i.e., hollow core, cell, holes or wells) of a honeycomb structure, a preformed sag-resistant nucleus-forming monolithic composite of incompatible in situ-expandable thermoplastic particles which form closed microcells upon isotropic expansion, in a thermosettable or thermoplastic matrix resin, which composite possesses an external shape and size that is dimensionally similar to the hollow interior portion of the honeycomb, heating the composite to a temperature which causes isotropic expansion of the in situ-expandable thermoplastic particles in the composite, and expansion of the composite within the interior portion honeycomb structure to achieve a faultless interface with the wall of the interior portion honeycomb structure.
Another embodiment of the invention involves the processes for forming the preformed sag-resistant nucleus-forming monolithic composites of incompatible in situ-expandable thermoplastic particles, which form closed-microcells upon isotropic expansion, in a thermosettable or thermoplastic matrix resin, of the invention. These monolithic composites may be made by extrusion, pultrusion or casting processes. In these processes, all of the components for making the monolithic composite are blended in conventional mixing equipment, with the exception of any continuous fibrous component that becomes a component of the monolithic composite. In the absence of any continuous fibrous component the blend of components may be extruded or casted to the desired monolithic composite shape. When continuous fibrous components are incorporated into the monolithic composite, it is desirable to make the monolithic composite by pultrusion processes. The desired length of the monolithic composite in the case of extrusion and pultrusion may be achieved by cutting the exudates with a knife or other cutting device. The length of the composite in the case of casting is determined by the size and shape of the mold used in casting.